Process for the production of acetic acid

ABSTRACT

A process for the production of acetic acid by carbonylating methanol and/or a reactive derivative thereof with carbon monoxide in at least one carbonylation reaction zone containing a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate, at least one promoter selected from ruthenium, osmium and rhenium and at least one catalyst system stabiliser selected from indium, cadmium, mercury, gallium and zinc and wherein the molar ratio of iridium:promoter:stabiliser in the liquid reaction composition is maintained in the range 1:(&gt;2 to 15):(0.25 to 12).

This application is the U.S. National Phase of International ApplicationPCT/GB2003/003834, filed 3 Sep. 2003, which designated the U.S.PCT/GB2003/003834 claims priority to British Application No. 0221800.6filed 19 Sep. 2002. The entire content of these applications areincorporated herein by reference.

The present invention relates to a process for the production of aceticacid and in particular to a process for the production of acetic acid bythe carbonylation of methanol and/or a reactive derivative thereof inthe presence of a promoted iridium catalyst.

The production of acetic acid by the carbonylation of methanol in thepresence of an iridium catalyst and a promoter such as ruthenium isdescribed, for example, in EP-A-0752406, EP-A-0849248, EP-A-0849249, andEP-A-1002785.

EP-A-0643034 describes a process for the carbonylation of methanoland/or a reactive derivative thereof in the presence of acetic acid, aniridium catalyst, methyl iodide, at least a finite concentration ofwater, methyl acetate and a promoter selected from ruthenium and osmium.

EP-A-0 749 948 describes a process for the carbonylation of an alkylalcohol such as methanol and/or a reactive derivative thereof to producethe corresponding carboxylic acid and/or ester in the presence of aniridium catalyst, an alkyl halide, water and at least one promoterselected from cadmium, mercury, zinc, gallium, indium and tungsten,optionally with a co-promoter selected from ruthenium, osmium andrhenium.

In a carbonylation process employing a promoted iridium catalyst, it hasgenerally been found that the higher the concentration of promoter, thegreater the rate of reaction. However, it has also been found that wherethe carbonylation process is carried out using relatively highconcentrations of promoter precipitation of the catalyst system (iridiumand promoter) may occur.

In addition, under certain operating conditions, such as during carbonmonoxide deficient conditions, precipitation of the catalyst system mayoccur.

Thus, there remains a need for an iridium-catalysed promotedcarbonylation process in which catalyst system stability is improvedand, in particular, an iridium-catalysed promoted carbonylation processin which catalyst system stability is improved and in which thecarbonylation rate is also at least maintained or increased.

The present invention solves the technical problem defined above byemploying at least one of indium, cadmium, mercury, zinc and gallium inthe liquid reaction composition.

Accordingly, the present invention provides a process for the productionof acetic acid by carbonylating methanol and/or a reactive derivativethereof with carbon monoxide in at least one carbonylation reaction zonecontaining a liquid reaction composition comprising an iridiumcarbonylation catalyst, methyl iodide co-catalyst, a finiteconcentration of water, acetic acid, methyl acetate, at least onepromoter selected from ruthenium, osmium and rhenium and at least onecatalyst system stabiliser selected from indium, cadmium, mercury,gallium and zinc and wherein the molar ratio ofiridium:promoter:stabiliser in the liquid reaction composition ismaintained in the range 1:(>2 to 15):(0.25 to 12).

The present invention further provides for the use of at least one ofindium, cadmium, mercury, gallium and zinc as a catalyst systemstabiliser in a process for the production of acetic acid which processcomprises carbonylating methanol and/or a reactive derivative thereofwith carbon monoxide in at least one carbonylation reaction zonecontaining a liquid reaction composition comprising an iridiumcarbonylation catalyst, methyl iodide co-catalyst, a finiteconcentration of water, acetic acid, methyl acetate, at least onepromoter selected from ruthenium, osmium and rhenium; and at least onecatalyst system stabilizer selected from indium, cadmium, mercury,gallium and zinc and wherein the molar ratio ofiridium:promoter:stabiliser in the liquid reaction composition ismaintained in the range 1:(>2 to 15):(0.25 to 12).

The present invention allows the stability of the catalyst system to beimproved whilst maintaining or increasing the carbonylation rate.

Advantageously, the present invention allows the process to be operatedat lower ratios of promoter:iridium, thereby reducing the amount ofexpensive promoter needed.

In addition, the present invention allows the process to be operated atlower iridium concentrations whilst at least maintaining thecarbonylation rate.

The reaction zone may comprise a conventional liquid-phase carbonylationreaction zone.

Preferably, two reaction zones are used, the first and second reactionzones being maintained in separate reaction vessels with means forwithdrawing from the first reaction vessel and passing to the secondreaction vessel liquid reaction composition from the first reactionvessel with dissolved and/or entrained carbon monoxide. Such a separatesecond reaction vessel may comprise a section of pipe between the firstreaction vessel and a liquid reaction composition flashing valve.Preferably the pipe is liquid full. Typically the pipe's length todiameter ratio may be about 12:1, though length to diameter ratios bothhigher and lower than this may be employed.

Typically, at least a portion of the liquid reaction compositiontogether with dissolved and/or entrained carbon monoxide is withdrawnfrom the first reaction zone and at least a portion of the withdrawnliquid and dissolved and/or entrained carbon monoxide passed to a secondreaction zone. Preferably substantially all the liquid reactioncomposition together with dissolved and/or entrained carbon monoxidewithdrawn from the first reaction zone is passed to the second reactionzone.

The pressure of the carbonylation reaction in the first reaction zone issuitably in the range 15 to 200 barg, preferably 15 to 100 barg, morepreferably 15 to 50 barg and yet more preferably 18 to 35 barg. Thetemperature of the carbonylation reaction in the first reaction zone issuitably in the range 100 to 300° C., preferably in the range 150 to220° C.

The second reaction zone may be operated at a reaction temperature inthe range 100 to 300° C., preferably in the range 150 to 230° C. Thesecond reaction zone may be operated at a temperature higher than thefirst reaction zone, typically up to 20° C. higher. The second reactionzone may be operated at a reaction pressure in the range 10 to 200 barg,preferably in the range 15 to 100 barg. Preferably, the reactionpressure in the second reaction zone is equal to or less than thereaction pressure in the first reaction zone. The residence time ofliquid reaction composition in the second reaction zone is suitably inthe range 5 to 300 seconds, preferably 10 to 100 seconds.

The carbon monoxide reactant for the carbonylation reactions may beessentially pure or may contain inert impurities such as carbon dioxide,methane, nitrogen, noble gases, water and C₁ to C₄ paraffinichydrocarbons. The presence of hydrogen in the carbon monoxide andgenerated in situ by the water gas shift reaction is preferably keptlow, for example, less than 1 bar partial pressure, as its presence mayresult in the formation of hydrogenation products. The partial pressureof carbon monoxide in the first and second reaction zones is suitablyindependently in the range 1 to 70 bar, preferably 1 to 35 bar and morepreferably 1 to 15 bar.

There may be introduced to the second reaction zone carbon monoxide inaddition to that introduced to the second reaction zone as dissolvedand/or entrained carbon monoxide. Such additional carbon monoxide may beco-joined with the first liquid reaction composition prior tointroduction to the second reaction zone and/or may be fed separately toone or more locations within the second reaction zone. Such additionalcarbon monoxide may contain impurities, such as for example H₂, N₂, CO₂and CH₄. The additional carbon monoxide may be comprised of highpressure off-gas from the first reaction zone which could advantageouslyallow the first reaction zone to be operated at a higher CO pressurewith the resulting higher flow of carbon monoxide being fed to thesecond reaction zone. Additionally it could eliminate the requirementfor a high pressure off-gas treatment.

The additional carbon monoxide may also be comprised of another carbonmonoxide-containing gas stream such as for example a carbonmonoxide-rich stream from another plant.

Preferably greater than 10%, more preferably greater than 25%, even morepreferably greater than 50%, for example at least 95%, of the dissolvedand/or entrained carbon monoxide in the withdrawn reaction compositionfrom the first reaction zone is consumed in the second reaction zone.

In the process of the present invention, suitable reactive derivativesof methanol include methyl acetate, dimethyl ether and methyl iodide. Amixture of methanol and reactive derivatives thereof may be used asreactants in the process of the present invention. Water is required asco-reactant for ether or ester reactants Preferably, methanol and/ormethyl acetate are used as reactants.

At least some of the methanol and/or reactive derivative thereof will beconverted to, and hence present as, methyl acetate in the liquidreaction composition by reaction with the carboxylic acid product orsolvent. Preferably, the concentrations of methyl acetate in the liquidreaction compositions in the first and second reaction zones areindependently in the range 1 to 70% by weight, more preferably 2 to 50%by weight, most preferably 3 to 35% by weight

Water may be formed in situ in the liquid reaction compositions, forexample, by the esterification reaction between methanol reactant andacetic acid product. Water may be introduced independently to the firstand second carbonylation reaction zones together with or separately fromother components of the liquid reaction compositions. Water may beseparated from other components of reaction compositions withdrawn fromthe reaction zones and may be recycled in controlled amounts to maintainthe required concentration of water in the liquid reaction compositions.Preferably, the concentrations of water in the liquid reactioncompositions in the first and second reaction zones are independently inthe range 0.1 to 20% by weight, more preferably 1 to 15% by weight, yetmore preferably 1 to 10% by weight.

Preferably, the concentration of methyl iodide co-catalyst in the liquidcarbonylation reaction compositions in the first and second reactionzones is independently in the range 1 to 20% by weight, preferably 2 to16% by weight.

The iridium catalyst in the liquid reaction compositions in the firstand second reaction zones may comprise any iridium-containing compoundwhich is soluble in the liquid reaction compositions. The iridiumcatalyst may be added to the liquid reaction compositions in anysuitable form which dissolves in the liquid reaction compositions or isconvertible to a soluble form. Preferably the iridium may be used as achloride free compound such as acetates which are soluble in one or moreof the liquid reaction composition components, for example water and/oracetic acid and so may be added to the reaction as solutions therein.Examples of suitable iridium-containing compounds which may be added tothe liquid reaction composition include IrCl₃, IrI₃, IrBr₃, [Ir(CO)₂I]₂,[Ir(CO)₂Cl]₂, [Ir(CO)₂Br]₂, [Ir(CO)₄I₂]⁻H⁺, [Ir(CO)₂Br₂]⁻H⁺,[Ir(CO)₂I₂]⁻H⁺, [Ir(CH₃)I₃(CO)₂]⁻H⁺, Ir₄(CO)₁₂, IrCl₃.4H₂O, IrBr₃.4H₂O,Ir₃(CO)₁₂, iridium metal, Ir₂O₃, IrO₂, Ir(acac)(CO)₂, Ir(acac)₃, iridiumacetate, [Ir₃O(OAc)₆(H₂O)₃][OAc], and hexachloroiridic acid H₂[IrCl₆],preferably, chloride-free complexes of iridium such as acetates,oxalates and acetoacetates.

Preferably, the concentration of the iridium catalyst in the liquidreaction compositions of the first and second reaction zones isindependently in the range 100 to 6000 ppm by weight of iridium.

The liquid reaction compositions in the first and second reaction zonesadditionally comprises one or more promoters. Suitable promoters areselected from ruthenium, osmium and rhenium, and are more preferablyselected from ruthenium and osmium. Ruthenium is the most preferredpromoter. The promoter may comprise any suitable promotermetal-containing compound which is soluble in the liquid reactioncomposition. The promoter may be added to the liquid reactioncomposition for the carbonylation reaction in any suitable form whichdissolves in the liquid reaction composition or is convertible tosoluble form.

Examples of suitable ruthenium-containing compounds which may be used assources of promoter include ruthenium (III) chloride, ruthenium (III)chloride trihydrate, ruthenium (IV) chloride, ruthenium (III) bromide,ruthenium metal, ruthenium oxides, ruthenium (III) formate,[Ru(CO)₃I₃]−H+, [Ru(CO)₂I₂]_(n), [Ru(CO)₄I₂], [Ru(CO)₃I₂]₂,tetra(aceto)chlororuthenium(II,III), ruthenium (III) acetate, ruthenium(III) propionate, ruthenium (III) butyrate, ruthenium pentacarbonyl,trirutheniumdodecacarbonyl and mixed ruthenium halocarbonyls such asdichlorotricarbonylruthenium (II) dimer, dibromotricarbonylruthenium(II) dimer, and other organoruthenium complexes such as tetrachlorobis(4-cymene)diruthenium(II), tetrachlorobis(benzene)diruthenium(II),dichloro(cycloocta-1,5diene) ruthenium (II) polymer andtris(acetylacetonate)ruthenium (III).

Examples of suitable osmium-containing compounds which may be used assources of promoter include osmium (III) chloride hydrate and anhydrous,osmium metal, osmium tetraoxide, triosmiumdodecacarbonyl, [Os(CO)₄I₂],[Os(CO)₃I₂]₂, [Os(CO)₃I₃]−H+, pentachloro-μ-nitrodiosmium and mixedosmium halocarbonyls such as tricarbonyldichloroosmium (II) dimer andother organoosmium complexes.

Examples of suitable rhenium-containing compounds which may be used assources of promoter include Re₂(CO)₁₀, Re(CO)₅Cl, Re(CO)₅Br, Re(CO)₅I,ReCl₃.xH₂O, [Re(CO)₄I]₂, Re(CO)₄I₂]⁻H⁺ and ReCl₅.yH₂O.

Preferably, the promoter is present in an effective amount up to thelimit of its solubility in the liquid reaction compositions and/or anyliquid process streams recycled to the carbonylation reactor from theacetic acid recovery stage. The promoter is suitably present in theliquid reaction compositions at a molar ratio of promoter to iridium of[greater than 2 to 15]:1, preferably [greater than 2 to 10]:1, morepreferably [4 to 10]:1. A suitable promoter concentration is less than8000 ppm, such as 400 to 7000 ppm.

The indium, cadmium, mercury, zinc and/or gallium catalyst systemstabiliser may comprise any indium, cadmium, mercury, zinc or galliumcontaining compound which is soluble in the liquid reactioncompositions. The catalyst system stabiliser may be added to the liquidreaction composition in the first and/or second reaction zone in anysuitable form which dissolves in the liquid reaction composition or isconvertible to a soluble form.

Examples of suitable indium-containing compounds which may be usedinclude indium acetate, InCl₃, InI₃, InI, In(OH)₃ and indiumacetylacetonate. Preferably, the indium-containing compound is indiumacetate or InI₃.

Examples of suitable cadmium-containing compounds which may be usedinclude Cd(OAc)₂, CdI₂, CdBr₂, CdCl₂, Cd(OH)₂, and cadmiumacetylacetonate.

Preferably, the cadmium-containing compound is cadmium acetate or CdI₂.

Examples of suitable mercury-containing compounds which may be usedinclude Hg(OAc)₂, HgI₂, HgBr₂, HgCl₂, Hg₂I₂, and Hg₂Cl₂. Preferably, themercury-containing compound is mercury acetate or HgI₂.

Examples of suitable zinc-containing compounds which may be used includeZn(OAc)₂, Zn(OH)₂, ZnI₂, ZnBr₂, ZnCl₂, and zinc acetylacetonate.Preferably, the zinc-containing compound is zinc acetate or ZnI₂.

Examples of suitable gallium-containing compounds which may be usedinclude gallium acetylacetonate, gallium acetate, GaCl₃, GaBr₃, GaI₃,Ga₂Cl₄ and Ga(OH)₃. Preferably, the gallium-containing compound isgallium acetate or GaI₃.

The molar ratio of catalyst system stabiliser:iridium in the liquidreaction compositions of the first and second reaction zones isindependently in the range (0.25 to 12):1, preferably (1 to 12):1, forexample (1 to 8):1

The molar ratio of iridium:promoter:catalyst system stabiliser in theliquid reaction compositions is independently in the range 1:(greaterthan 2 to 15):(0.25 to 12). Suitably, the molar ratio ofiridium:promoter:catalyst system stabiliser may be 1:(greater than 2 to10):(0.25 to 12), such as 1:(greater than 2 to 10):(1 to 12), forexample, 1:(3 to 10):(0.25 to 12), 1:(4 to 10):(0.25 to 12), 1:(4 to10):(1 to 12), 1:(4 to 10):(1 to 8) and preferably, 1:(3 to 10):(1 to10), 1:(greater than 4 to 10):(1 to 10), especially, 1:(greater than 4to 10):(1 to 8).

In a preferred embodiment of the present invention, the promoter isruthenium and the molar ratio of iridium:ruthenium:catalyst systemstabiliser in the liquid reaction compositions is independently in therange 1:(greater than 2 to 15):(0.25 to 12). Suitably, the molar ratioof iridium:ruthenium:catalyst system stabiliser may be 1:(greater than 2to 10):(0.25 to 12), such as 1:(greater than 2 to 10):(1-12), forexample, 1:(4 to 10):(0.25 to 12), 1:(4 to 10):(1 to 12), 1:(4 to 10):(1to 8) and preferably, 1:(greater than 4 to 10):(1 to 10), especially,1:(greater than 4 to 10):(1 to 8).

A suitable catalyst system stabiliser concentration in the liquidreaction compositions of the first and second reaction zones isindependently less than 9000 ppm, such as 300 to 8000 ppm, for example300 to 5000 ppm.

Preferably, the iridium, promoter and the indium, cadmium, mercury,gallium and/or zinc-containing compound are free of impurities whichprovide or generate in-situ ionic iodides which may inhibit thereaction, for example, alkali or alkaline earth metal or other metalsalts.

Ionic contaminants such as, for example, (a) corrosion metals,particularly nickel, iron and chromium and (b) phosphines or nitrogencontaining compounds or ligands which may quaternise in-situ, should bekept to a minimum in the liquid reaction composition as these will havean adverse effect on the reaction by generating I⁻ in the liquidreaction composition which has an adverse effect on the reaction rate.Similarly, contaminants such as alkali metal iodides, such as lithiumiodide, should be kept to a minimum. Corrosion metal and other ionicimpurities may be reduced by the use of a suitable ion-exchange resinbed to treat the reaction composition or preferably the catalyst recyclestream. Preferably, ionic contaminants are kept below a concentration atwhich they would generate 500 ppm I⁻, preferably less than 250 ppm I⁻ inthe liquid reaction composition.

Acetic acid product may be recovered from the second reaction zone andoptionally together with or separately from the first reaction zone byflash separation. In flash separation liquid reaction composition ispassed to a flashing zone via a flashing valve. The flash separationzone may be an adiabatic flash vessel or may have additional heatingmeans. In the flash separation zone a liquid fraction comprising themajority of the iridium catalyst and the majority of the promoter andstabiliser salt is separated from a vapour fraction comprising aceticacid, carbonylatable reactant, water and methyl iodide carbonylationco-catalyst and non-condensable gases such as nitrogen, carbon monoxide,hydrogen and carbon dioxide; the liquid fraction being recycled to thefirst reaction zone and the vapour fraction being passed to one or moredistillation zones. In a first distillation zone acetic acid product isseparated from the light components (methyl iodide and methyl acetate).The light components are removed overhead, and recycled to the firstand/or second reaction zones. Also removed overhead is a low pressureoff-gas comprising the non-condensable gases such as nitrogen, carbonmonoxide, hydrogen and carbon dioxide. Such a low-pressure off-gasstream may be passed through an off-gas treatment section to removecondensable materials such as methyl iodide, prior to being vented toatmosphere, for example, via a flare.

The acetic acid produced by the process according to the presentinvention may be further purified by conventional processes, for examplefurther distillation to remove impurities such as water, unreactedcarbonylation reactant and/or ester derivative thereof andhigher-boiling by-products.

The process of the present invention may be performed as a batch or as acontinuous process, preferably as a continuous process.

The present invention will now be illustrated by way of example only andwith reference to the following Examples.

General Reaction Method

All experiments were performed in either a 300 cm³ zirconium or a 300cm³ Hastelloy autoclave, equipped with a stirrer and a liquid injectionfacility. Ruthenium acetate solution (18.7 g, approximately 5 wt %ruthenium), a catalyst system stabiliser compound (when used) and aceticacid (approx. 10.0 g) were placed into the autoclave base. The autoclavewas pressure tested to 32 barg with nitrogen, flushed twice withnitrogen at 20 barg and then three times with carbon monoxide up to 10barg. An initial charge consisting of methyl acetate (approx 48.0 g)acetic acid (approx 34.0 g), methyl iodide (approx 13.3 g) and water(approx 11.0 g) was placed into the autoclave, which was then repurgedwith carbon monoxide and vented slowly to prevent loss of volatiles.

Carbon monoxide (8 barg) was fed into the autoclave which was thenheated, with stirring (1500 rpm) to 190° C. The catalyst injectionsystem was primed with approx 6.3 g of iridium acetate solution (approx.5 wt % iridium) and acetic acid (approx 8.7 g) and injected with anoverpressure of carbon monoxide to bring the autoclave pressure to 28barg.

The reaction rate was monitored by drop in carbon monoxide pressure froma ballast vessel, typically pressured to 82 barg. The autoclave wasmaintained at a constant temperature of 190° C. and pressure of 28 bargthroughout the reaction. After uptake of carbon monoxide from theballast vessel had ceased the autoclave was isolated from the gas supplyand cooled. After cooling, a gas analysis sample was taken, and theautoclave vented. The liquid components were discharged, and analysedfor liquid by-products by known established gas chromatography methods.Detected components were quantified by integration of the componentpeaks relative to an external standard and expressed as parts permillion (ppm) by weight. The major product in each of the batchcarbonylation experiments was acetic acid.

The rate of gas uptake at a certain point in a reaction run was used tocalculate the carbonylation rate, as number of moles of reactantconsumed per litre of cold degassed reactor composition per hour(mol/l/h) at a particular reactor composition (total reactor compositionbased on a cold degassed volume)

The methyl acetate concentration was calculated during the course of thereaction from the starting composition, assuming that one mole of methylacetate was consumed for every mole of carbon monoxide that wasconsumed. No allowance was made for organic components in the autoclaveheadspace.

Catalyst System Stability Test

On completion of the carbonylation reaction (that is when no carbonmonoxide gas uptake could be observed), the reaction solution wasallowed to cool to room temperature. The autoclave was thendepressurized and a 25 ml sample of cooled reaction solution wastransferred from the autoclave to a Fischer-Porter tube. The tube wasthen sealed and pressurized with nitrogen to 0.5 barg and heated withstirring to 130° C. for 5 hours before cooling to room temperature andventing.

The formation or otherwise of a precipitate was determined by visualinspection of the tube contents.

EXAMPLES Experiment A

A baseline experiment was performed with the autoclave charged withmethyl acetate (47.96 g) acetic acid (44.1 g) ruthenium acetate solution(18.7 g) water (11.0 g) methyl iodide (12.59 g). The catalyst solutionconsisted of an iridium solution (6.31 g) with acetic acid (8.7 g). Theapproximate ratio of iridium to ruthenium was 1:6. The rate of reactionat a calculated reaction composition of 12% methyl acetate and catalystsystem stability results are shown in Table 1.

Experiment B

A baseline experiment was performed with the autoclave charged withmethyl acetate (48.01 g) acetic acid (43.1 g) ruthenium acetate solution(6.2 g) water (13.24 g) methyl iodide (13.34 g). The catalyst solutionconsisted of an iridium solution (6.31 g) with acetic acid (8.72 g). Theapproximate ratio of iridium to ruthenium was 1:2. The rate of reactionat a calculated reaction composition of 12% methyl acetate and catalystsystem stability results are shown in Table 1.

Example 1

Experiment A was repeated except that the autoclave was also chargedwith 0.86 g InI₃. The rate of reaction at a calculated reactioncomposition of 12% methyl acetate and catalyst system stability resultsare shown in Table 1.

Example 2

Experiment A was repeated except that the autoclave was also chargedwith 1.73 g of InI₃. The rate of reaction at a calculated reactioncomposition of 12% methyl acetate and catalyst system stability resultsare shown in Table 1.

Example 3

Experiment A was repeated except that the autoclave was also chargedwith 0.51 g of In(OAc)₃. The rate of reaction at a calculated reactioncomposition of 12% methyl acetate and catalyst system stability resultsare shown in Table 1.

Example 4

Experiment A was repeated except that the autoclave was charged with6.92 g of ruthenium solution and 1.013 g of In(OAc)₃. The main charge ofthe autoclave was adjusted to 48 g of methyl acetate, 44.9 g of aceticacid, 13.7 g of water and 13.3 g of methyl iodide. The catalyst solutionconsisted of an iridium solution (3.18 g) with acetic acid (8.7 g). Theiridium to ruthenium to indium ratio was 0.5:2:2; however, the indiumconcentration was half that of Experiment B. The rate of reaction at acalculated reaction composition of 12% methyl acetate and catalystsystem stability results are shown in Table 1.

TABLE 1 Rate at 12 Ir:Ru:In wt % MeOAc Precipitate Experiment Molarratio mol/l/h formed Experiment A 1:6  24 Yes Example 1 1:6:1 27 NoExample 2 1:6:2 29 No Example 3 1:6:1 26.5 No Experiment B 1:2  19 NoExample 4 0.5:2:2  19 NoIn Table 1, it can be seen that from a comparison of Experiment A (noindium present) with Examples 1-3 (indium present) that both catalyststability and carbonylation rates are improved in Examples 1-3. It canalso be seen that from a comparison of Experiment B (no indium present)with Example 4 (indium present and a reduction in iridium concentration)that the carbonylation rate is maintained in Example 4.

1. A process for the production of acetic acid comprising carbonylatingmethanol and/or a reactive derivative thereof selected from the groupconsisting of methyl acetate, dimethyl ether and methyl iodide withcarbon monoxide in at least one carbonylation reaction zone containing aliquid reaction composition comprising an iridium carbonylationcatalyst, methyl iodide co-catalyst, a finite concentration of water,acetic acid, methyl acetate, at least one promoter selected fromruthenium, osmium and rhenium and at least one catalyst systemstabiliser selected from indium, cadmium, mercury, gallium and zinc andwherein the molar ratio of iridium:promoter:stabiliser in the liquidreaction composition is maintained in the range 1:(>2 to 15):(0.25 to12).
 2. A process according to claim 1 wherein the molar ratio ofiridium:promoter:stabiliser in the liquid reaction composition ismaintained in the range 1:(>2 to 10):(1 to 12).
 3. A process accordingto claim 1 wherein the molar ratio of iridium:promoter:stabiliser in theliquid reaction composition is maintained in the range 1:(3 to 10):(1 to10).
 4. A process according to claim 1 or claim 2 wherein theconcentration of catalyst system stabiliser in the liquid reactioncomposition is less than 9000 ppm.
 5. A process according to claim 1 orclaim 2 wherein the catalyst system stabiliser is selected from thegroup consisting of iodides or acetates of indium, cadmium, mercury,gallium and zinc.
 6. A process according to claim 1 or claim 2 whereinthe promoter is ruthenium.
 7. A process according to claim 1 or claim 2wherein the concentration of promoter in the liquid reaction compositionis less than 8000 ppm.
 8. A process according to claim 1 or claim 2wherein the concentration of water in the liquid reaction composition isin the range 0.1 to 20 wt %.
 9. A process according to claim 1 or claim2 wherein the carbonylation is carried out in two reaction zones.
 10. Aprocess according to claim 3 wherein the concentration of catalystsystem stabiliser in the liquid reaction composition is less than 9000ppm.
 11. A process according to claim 3 wherein the catalyst systemstabiliser is selected from the group consisting of iodides or acetatesof indium, cadmium, mercury, gallium and zinc.
 12. A process accordingto claim 3 wherein the promoter is ruthenium.
 13. A process according toclaim 3 wherein the concentration of promoter in the liquid reactioncomposition is less than 8000 ppm.
 14. A process according to claim 3wherein the concentration of water in the liquid reaction composition isin the range 0.1 to 20 wt %.
 15. A process according to claim 3 whereinthe carbonylation is carried out in two reaction zones.